Permeable membrane processes and systems are known in the art and have been employed or considered for a wide variety of gas and liquid separations. In such operations, a feed stream is brought into contact with the surface of a membrane, and the more readily permeable component of the feed stream is recovered as a permeate stream, with the less-readily permeable component being withdrawn from the membrane system as a non-permeate stream. Membrane separation modules are maintained at operating conditions which result in a non-permeate side pressure at which the feed gas is introduced and the non-permeate stream is withdrawn, and a permeate side pressure at which the permeate stream is withdrawn. The pressure on the non-permeate side of the membrane is higher than the pressure on the permeate side, and the pressure differential between the non-permeate and the permeate sides of the membrane generally determines the degree of separation attained by the membrane separation.
Membranes are widely used to separate permeable components from gaseous feed streams. Examples of such process applications include removal of acid gases from natural gas streams, removal of water vapor from air and light hydrocarbon streams, and removal of hydrogen from heavier hydrocarbon streams. Membranes are also employed in gas processing applications to remove permeable components from a process gas stream. Natural gas as produced from a gas well presents a separations challenge. Often the natural gas is found together with other components such as sulfur compounds, water, and associated gases. The associated gases found in natural gas streams typically include carbon dioxide, hydrogen sulfide, nitrogen, helium, and argon. Generally, these other gas components are separated from the natural gas by bulk methods employing membrane systems.
The inherent simplicity of such fluid separation operations constitutes an incentive to expand the use of membrane systems in commercial operations. In this regard, it will be appreciated that the selectivity and permeability characteristics of such membrane systems must be compatible with the overall production requirements of a given application. It is also necessary, of course, that the membranes exhibit acceptable stability and do not suffer undue degradation of their performance properties in the course of practical commercial operations.
Membranes for gas processing typically operate in a continuous manner, wherein a feed gas stream is introduced to the membrane gas separation module on a non-permeate side of a membrane. The feed gas is introduced at separation conditions which include a separation pressure and temperature which retains the components of the feed gas stream in the vapor phase, well above the dew point of the gas stream, or the temperature and pressure condition at which condensation of one of the components might occur. However, if the flow of the feed gas stream is interrupted, or the feed pressure is suddenly reduced, the residual material within the membrane separation zone could reach its dew point and condensation would then result. The feed gas stream fed to the gas separation membrane may contain a substantial amount of moisture and this moisture and other impurities may cause corrosion, condensation or other damage to instrumentation, piping, pneumatic tools, ventilators and other equipment associated with the gas separation membrane. In certain instances, it may also lead to inferior performance of the gas separation membrane or other equipment, such as adsorption traps. In order to compensate for damage caused by condensation occurring during shut-down of the feed gas stream during the lifetime of a membrane system, such membrane systems are often oversized to compensate for the loss of membrane surface over the useful life of the membrane. However, for high volume gas treating application, this over design of membrane capacity can be very costly, adding millions of dollars to the cost of a membrane system.
In gas drying applications, methods have been disclosed for employing sweep gases to remove moisture from the membrane before it condenses. For example, in air separation applications, which constitute a highly desirable field of use for permeable membranes, oxygen is typically the more readily permeable component of the feed air for particular membranes and is withdrawn as the permeate gas. In such embodiments, nitrogen is the less-readily permeable component and is recovered as non-permeate gas. Liquid water is generally removed from feed air upstream of the membrane by conventional means such as knockout drums. Generally, any water vapor present in the feed air will permeate the membrane resulting in a dry non-permeate gas. In air separation applications, it has been found that the performance characteristics of the membranes are sensitive to the presence of certain contaminants in the feed air stream. Exposure to such contaminants may result in a significant reduction in the permeability of the membrane in use. Fortunately, most contaminants commonly present in ambient air, such as light hydrocarbons, H2O, and CO2, have been found to result in, at most, a modest decrease in membrane permeability. However, heavier hydrocarbons provide a not so benign impact upon a membrane system. The presence of even relatively low concentrations, e.g., less than 1 ppm by volume of C10+, of heavy hydrocarbon oil vapors, such as might enter the feed air stream from an oil lubricated air compressor, can result in rapid and extensive loss of membrane permeability.
In response to such an undesirable decrease in membrane permeability, it is a common practice to increase the size of the active membrane surface area with a safety factor sufficiently large to compensate for the anticipated permeability loss from all sources. Initially, therefore, the membrane system is significantly oversized for the desired product flow, and the feed gas compressor is typically operated in a turndown mode. As permeability degradation of the membrane proceeds, either the operating temperature or pressure, or both, are increased to compensate for the decrease in permeability. Typical membrane systems consist of multiple membrane modules. In some instances, it is necessary or desirable to bypass some of the modules in the membrane system initially so as to reduce excess membrane area employed when the membranes are new and exhibit their full permeability capability and then subsequently to bring such by-passed modules on stream as degradation of the initially employed modules progresses. In such instances, it will be appreciated that, in addition to a significant capital cost penalty associated with the provision of extra membrane surface area and extra membrane modules, such a membrane system must operate over a significant portion of its operating life under off design conditions and the control strategy for such a membrane system is more complex than for a system operating closer to optimum design conditions.
As an alternative to such over designed membrane systems to compensate for degradation in use, attempts have been made to restore lost performance, but such efforts were initially unsuccessful in developing an economically feasible means for restoring the permeability of degraded membranes. Restoring any portion of the degraded membranes would require interruption of the gas treating operation, displacing large quantities of gas. Replacement of degraded membranes is an expensive alternative. Neither over design of the membrane system nor interruption of gas product operations for membrane restoration treatment, or a combination of these approaches is an entirely satisfactory means for overcoming permeability degradation in practical commercial air or other gas separation operations. Further improvement in the response to the problem of membrane degradation is highly desirable.
U.S. Pat. No. 4,881,953 to Prasad et al. discloses an approach to the problem of preventing premature loss of membrane capacity by passing the feed gas mixture through a bed of adsorbent material, such as activated carbon to adsorb contaminants such as heavier hydrocarbon contaminants without the removal of lighter hydrocarbons. Prasad requires that a means for removing moisture from the feed gas be provided because high moisture levels generally limit the ability of activated carbon adsorbents to retain their adsorptive capacity for heavy hydrocarbons.
U.S. Pat. No. 5,030,251 to Rice et al. relates to the operation of a membrane separator which removes water vapor from a moist air feed to produce a drier air product. When such a membrane operation is stopped, some residual water vapor remains in the membrane separator and when the feed flow is resumed, the residual water vapor flows out with the non-permeate stream. This results in a less dry product produced during restarts than during the steady-state operation of the membrane separator. To correct this problem, a portion of the non-permeate product is saved in a storage tank and supplied to the membrane separation at a time when the feed is not being supplied to the separator to purge the residual water vapor between cycles. As disclosed, when the feed cycle is off, the air pressure of the non-permeate side of the separator reduces to atmospheric pressure. Then, because the pressure is in the storage tank is greater than atmospheric, some of the stored non-permeate bleeds back to form the purge stream.
U.S. Pat. No. 5,383,956 to Prasad et al., relates to processes and apparatus for starting up and shutting down membrane gas separation systems treating a wet gas feed gas stream. The process of Prasad et al. employs a membrane dryer module and a gas separation membrane module in various startup sequences and shut-down sequences for drying and separating the feed gas stream. In the shut-down of Prasad et al. which comprises at least one gas separation module and at least one membrane dryer, the flow of the feed gas is stopped to both membrane modules, and the modules are depressurized by removing pressurized gas from the non-permeate sides of the modules. The pressurized gas is allowed to permeate through the respective membrane modules to the permeate sides, followed by purging both the permeate and non-permeate sides of the membrane modules with a dry gas stream.
U.S. Pat. No. 5,669,959 to Doshi et al. provided for a purge stream to be passed to the non-permeate side of the membrane separation zone to remove a residual gas stream and thereby prevent condensation of the less-readily permeable, condensable component of the gas stream. Doshi et al. employs an adsorbent bed or zone to remove the condensable components of the gas stream to be used as the purge stream. While this has been found to be one method to protect the membrane, less expensive means are still being sought that do not involve the need to replace or regenerate an adsorbent bed.
When a natural gas stream is processed in a membrane separation zone, the presence of heavy hydrocarbons, such as C6+ hydrocarbons, and particularly C10+ hydrocarbons under certain conditions such as reduction of temperature and pressure, or a change in composition can result in the loss of membrane capacity and often permanent damage to the membrane. Processes are sought to prevent such damage to the membrane separation unit beyond those taught by the prior art.
It is an object of the invention, therefore, to provide an improved membrane system and process for overcoming the problem of degradation of permeability during hydrocarbon gas production operations such as in natural gas production.
It is another object of the invention to provide a membrane system and process obviating the need for significant over design or for premature replacement of degraded membrane modules.
It is a further object of the invention to provide a membrane system and process for maintaining membrane permeability and minimizing the need for the interruption of gas producing operations for the treatment of membrane modules for restoration of the permeability characteristics thereof.
It is another object of the invention to provide an inexpensive process for overcoming the degradation of permeability during hydrocarbon gas production operations such as natural gas production.